Directional condenser electret microphone

ABSTRACT

A miniature condenser microphone incorporates an assembly of a backplate, a diaphragm and an electrical circuit board mounted within a housing to divide the housing into front and back air volumes. The front air volume is adjacent the diaphragm and communicates with a sound inlet port. The rear air volume is adjacent the backplate and communicates with a working gap between the diaphragm and the backplate through a damping constriction formed by the backplate and the electrical circuit board.

United States Patent 1191 B Mir/W360 1111 3,895,194

Fraim July 15, 1975 [54] fi iiggggg CONDENSER ELECTRET FOREIGN PATENTS OR APPLICATIONS 879,857 3/1943 France l79/lll [75] Inventor: Freeman W. Fraim, Lexington,

Mass- Primary Examiner-Kathleen l-l. Claffy [73] Assignee: Thermo Electron Corporation, Assistant Examiner'c'eorge Stellar waltham, Mass Attorney, Agent, or FirmJames L. Neal [22] Filed: May 29, 1973 57 ABSTRACT [2]] Appl. NO- 36 ,9 A miniature condenser microphone incorporates an assembly of a backplate, a diaphragm and an electrical [52] Us. Cl l79/l21 R; 179M H E; 179/121 D circuit board mounted within a housing to divide the s 1 Int. Cl H04r 1/38; H04r 19/04 hwsmg 3 and volumes- The from air 58 Field of Search 179/111 R, 111 E, 121 D, Wlume adlecem dlaphragm P cmmu,nicat es 179/l DM, 178 179 307/88 ET with a sound Inlet port. The rear air volume is ad acent the backplate and communicates with a working gap between the diaphragm and the backplate through [56] References Cited a damping constriction formed by the backplate and UNITED STATES PATENTS the electrical circuit board. 3,300,585 H1967 Reedyk et a1. 179/111 R 3,777,079 12/1974 Fischer et al. 179/121 D 4 Clams, 2 Drawing Flgures i 1 A /l/ y 63 1 1 74 78 76 16 24 56 34 32 6O PMENTEQJUL 15 1915 24 5e 34 32 m gg 1 DIRECTIONAL CONDENSER ELECTRET MICROPHONE c oss REFERENCE TO RELATED APPLICATION This application discloses subject matter similar to that appearing in application Ser. No. 364,997 filed May 29, 1973 for Unidirectional Condenser Microphone in the names of Freeman W. Fraim, Preston V. Murphy and Alan P. Woodward.

BRIEF SUMMARY OF THE INVENTION This invention provides a miniature condenser microphone of improved construction which is characterized by a high standard of performance and low production cost. A backplate having a diaphragm mounted on and spaced from one side thereof and an'electrical circuit board mounted on the other side forms a compact assembly which, when installed in a housing, forms a front air volume adjacent the diaphragm and'a back air volume adjacent the circuit board. A groove in the surface of the backplate adjacent the electrical circuit board establishes a fluid communication path between the space intermediate the diaphragm and the backplate and the rear air volume. The rear air volume acts as a resonance chamber. The groove and the electrical circuit board cooperate to form a constriction in the fluid communication path to damp the diaphragm and provide the desired performance characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cut-away, perspective view showing a preferred embodiment of the invention;

FIG. 2 is an exploded perspective view of the appara tus as shown in FIG. I.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIGS. l and 2, the miniature microphone incorporates a housing 12 comprised of an electrically conductive front member 14 and a rear member 16. In a preferred embodiment. the miniature housing 12 measures slightly less than 0.10 inches in thickness and has a face area of approximately 0.22 X 0.31 inches. Within the housing 10 is mounted an assembly 18 comprising a backplate 20, a diaphragm 22 and a board 24 having an electrical circuit thereon.

The backplate 20, formed of injection molded thermoplastic material, has a peripheral ridge 26 extending from an obverse face. The diaphragm 22 is mounted on the ridge 26and spaced from the backplate by a distance equal to the height of the ridge 26. To avoid contact between the backplate and the diaphragm during operation of the microphone, a pair of posts 28 are provided. The posts approximately equal or slightly exceed the height of the ridge 26. There is formed, between the backplate and the diaphragm, a working gap 30. In the reverse side of the backplate is a recess 32 for receiving the circuit board 24. The configuration of the recessed area conforms substantially to the configuration of the circuit board, except that along one portion thereof a portion of the recess 32 extends beyond the board 24 to form an open area 34. The circuit board 24 is securely bonded to the backplate 22 by an epoxy resin 66. A plurality of holes 36 extend through the backplate and communicate between the working gap and the recess area 32. A system of grooves 38 in the reverse side of the backplate 20 connects the holes 36 and extends into the open area 34. The epoxy resin 66 is positioned not to block the constriction formed by the grooves 38 which connect the holes 36 with the open area 34. A thin coating 40 of electrically conductivematerial, such as gold, renders the backplate electrically conductive;

The diaphragm 22 comprises an electret 42 having thereon a coating 44 of electrically conductive material, such as gold. The electret is described in US. Pat. No. 3,6l2,778 titled ElectretAcoustic Transducer and Method-of Making. Thediaphragm is mounted on the backplate by firmly securing the electret along the ridge 26 with the coating 44 facing away from the backplate 20.

The circuit board 24 is mounted in the recess 32with the side bearing electrical components means 46 facing away from the backplate. A relatively smooth side 48 seats within the recess 34. A plurality of leads 50 extend from the circuit board for connection to a terminal board 52 mounted on the housing 12.

The assembly 18 is mounted within the housing 12 to form a first or front air chamber 54 adjacent the front housing member 14 and a second or rear air chamber 56 adjacent the rear housing member 16. The spacing is accomplished by a ring 58 and a resilient gasket 60. The ring 58 is plastic with an electrically conductive coating 62. It establishes electrical connection between the diaphragm and the front member 14. The resilient gasket 60 spaces the assembly 18 from the rear member 16 and loads the assembly so that it is pressed towards the front member 14.

Means 68 extends from the front housing member 14 to form a front sound inlet port 70, communicating with front air chamber 54 through an opening 64 in the ring 58.

During operation of the microphone, the rear air chamber 56 acts as a resonance chamber. It is in fluid communication with the working gap 30 through the holes 36 and the grooves 38. The grooves 38 and the surface 48 of the circuit board 24 form a air passageway which constitutes a constriction in the fluid communication path between the working gap 30 and the rear air chamber 56 to damp the diaphragm 22. The character of the passageway formed by the grooves 38 and the surface 48 determines the amount of damping which occurs. It will be appreciated that the size of the grooves as well as its length are the fundamental parameters which may conveniently be designed to provide a desired damping action.

Optionally, the microphone 10 may have a second sound inlet port 74 to provide a directional characteristic. Port 74 alone will establish essentially bidirectional operation. If the port is covered by acoustical resistance material 80, spaced from rear member 16 by spacer 76 having opening 78 therein, a unidirectional characteristic preferential to the front is established.

This invention has been described with reference to a preferred embodiment. It will be understood by those skilled in the art that changes may be made without departing from the scope of the invention.

I claim:

1. A miniature condenser microphone including a housing enclosing an electret diaphragm and a minia ture electrical circuit to electrically connect the dia phragm with means external of said housing, said microphone comprising:

a perforate electrically conductive backplate having obverse and reverse sides,

' said obverse side having said d iaph ragm, mounted thereover and spaced therefromt'o farm-2i 'orking gap between said diaphragm and the byerse side of said backplate, said backplate inclildin'g at least one air passageway mean's. there'through to permit fluid communication between the obverse and reverse sides of said backplate; an imperforate electrical circuit board supporting said miniature electrical circuit, said board being mounted on the reverse side of said backplate confronting all said air passageway means along the reverse side of said backplate and partially exposing the reverse side of said backplate, said backplate mating with said board to form a constrictive passageway along the confronting surfaces thereof, said constrictive passageway having an open end at an edge of said board confronting said backplate; said backplate, diaphragm and circuit board forming an assembly; means mounting said assembly in said housing for dividing the interior thereofinto a first air space adjacent said diaphragm along the surface thereof facing away from said working gap and a second air space adjacent said circuit board, said second air space communicating with said working gap through said open end, said constrictive channel and said air passageway means; and 7 means forming in said housing a sound inlet port to said first air space. 2. A miniature condenser microphone according to claim 1 wherein said air passageway means comprises means forming at least one hole in said backplate and wherein said constrictive passageway is formed by a groove extending from said hole between the reverse side of said backplate and said circuit board.

3. A miniature condenser microphone according to claim 1 wherein said air passageway means comprises means forming a plurality of holes in said backplate and wherein said constrictive passageway is formed by the combination of said circuit board and groove means along the reverse side of said backplate joining said holes, said circuit board fronting a part of said groove means.

4. A miniature condenser microphone according to claim 3 further comprising a recess in the reverse side of said backplate for receiving said circuit board. 

1. A miniature condenser microphone including a housing enclosing an electret diaphragm and a miniature electrical circuit to electrically connect the diaphragm with means external of said housing, said microphone comprising: a perforate electrically conductive backplate having obverse and reverse sides, said obverse side having said diaphragm mounted thereover and spaced therefrom to form a working gap between said diaphragm and the obverse side of said backplate, said backplate including at least one air passageway means therethrough to permit fluid communication between the obverse and reverse sides of said backplate; an imperforate electrical circuit board supporting said miniature electrical circuit, said board being mounted on the reverse side of saiD backplate confronting all said air passageway means along the reverse side of said backplate and partially exposing the reverse side of said backplate, said backplate mating with said board to form a constrictive passageway along the confronting surfaces thereof, said constrictive passageway having an open end at an edge of said board confronting said backplate; said backplate, diaphragm and circuit board forming an assembly; means mounting said assembly in said housing for dividing the interior thereof into a first air space adjacent said diaphragm along the surface thereof facing away from said working gap and a second air space adjacent said circuit board, said second air space communicating with said working gap through said open end, said constrictive channel and said air passageway means; and means forming in said housing a sound inlet port to said first air space.
 2. A miniature condenser microphone according to claim 1 wherein said air passageway means comprises means forming at least one hole in said backplate and wherein said constrictive passageway is formed by a groove extending from said hole between the reverse side of said backplate and said circuit board.
 3. A miniature condenser microphone according to claim 1 wherein said air passageway means comprises means forming a plurality of holes in said backplate and wherein said constrictive passageway is formed by the combination of said circuit board and groove means along the reverse side of said backplate joining said holes, said circuit board fronting a part of said groove means.
 4. A miniature condenser microphone according to claim 3 further comprising a recess in the reverse side of said backplate for receiving said circuit board. 